Ectopic expression of the intracellular domain of NOTCH-4/INT3 leads to tumorigenesis in the mouse mammary gland. This results from a gain-of- function mutation. To evaluate gain-of-function NOTCH-4/INT3 activity in human cancers we have surveyed human breast, lung, and colon carcinoma tissue culture cell lines for evidence of increased NOTCH- 4/INT3 RNA expression. High levels of a 1.8 Kb NOTCH-4/INT3 RNA species are detected in normal human testis but not in other tissues where a 6.5 kb species is prevalent. Transformed human cancer cell lines express the 1.8 Kb NOTCH-4/INT3 RNA species. We have shown that this RNA species encodes a truncated form of the NOTCH-4/INT3 intracellular domain (ICD). This novel NOTCH-4/INT3 protein includes the CDC10 repeats and amino acid residues C-terminal to them, but is missing the CBF-1 binding region of the NOTCH-4/INT3 ICD. This suggests that it has a different mode of action. Furthermore, we have shown that a transgene which expresses the 1.8 Kb NOTCH-4/INT3 RNA species in the normal human mammary epithelial cell line MCF-10A enables these cells to grow in soft agar. The mouse mammary tumor virus (MMTV) has been shown to integrate frequently into INT-6 in MMTV induced mouse mammary tumors. The INT6 gene has been highly conserved through evolution and has recently been shown to encode the p48 component of the eucaryotic translation initiation factor 3 (eIF-3) complex. We have obtained recombinant DNA clones of the Drosophila eIF-3p48/INT-6. The gene is comprised of three exons within 1.8 Kb of genomic DNA located at cytogenetic position 73C2 in the Drosophila genome. The 1.5 Kb eIF- 3p48/INT-6 RNA species encodes a protein composed of 364 amino acid residues whose sequence is 71% similar to that of the mouse/human eIF- 3/INT-6 amino acid sequence. eIF-3p48/INT-6 RNA is expressed throughout development in Drosophila and the encoded protein is associated with the microsomal subcellular fraction. - Breast cancer model, Int 6, Mammary tumorigenesis, Mouse mammary tumor virus, NOTCH 4/Int 3,